Conventionally, TIG welding (tungsten inert gas welding) which uses a double-shielding structure type welding torch has been performed, and the welding torch includes a tungsten electrode, a first shield nozzle which surrounds the tungsten electrode, and a second shield nozzle which is placed at the outside of the first shield nozzle, wherein a first shield gas is supplied from the inner side of the first shield nozzle, and a second shield gas is supplied from an area between the first shield nozzle and the second shield nozzle. (For example, refer to Patent documents 1 to 3.)
Generally, argon is used in TIG welding as a shield gas from the viewpoint of molten pool protection.
In Patent document 1, the TIG welding method described below is disclosed. In the method, a tungsten electrode of a welding torch is inserted into a groove of an object to be welded having the groove, and voltage is applied to an energization part of the tungsten electrode while flowing a shielding gas so that a molten pool is formed due to a welding arc generated between the tungsten electrode and the object to be welded. Then, a welding wire, which is fed out from the welding torch, is entered in the generated molten pool, and the welding torch is moved to perform welding. In the welding, an inner shielding gas and an outer shielding gas are used. The inner shielding gas is fed from the circumference of the tungsten electrode toward the terminal end of the electrode. The outer shielding gas consists of two flows, wherein one flow is fed from the outside of the inner shield gas toward a groove to prevent the oxidation of the molten pool, and the other flow is fed to prevent the entrapment of oxygen, which is included in air supplied from the outside, into the welding article.
Furthermore, Patent document 1 discloses that argon gas including 2 to 10% by volume of hydrogen or argon gas including 10 to 80% by volume of helium is used as the inner shield gas and the outer shield gas to increase penetration depth.
Patent document 2 discloses a double-shielded-type TIG welding method wherein the welding is performed with a first shield gas which surrounds a tungsten electrode and with a second shield gas which surrounds the first shield gas. In the method, the welding is performed by using helium gas as the first shield gas at the flow rates of 6 to 10 L/min and using argon gas as the second shield gas at the flow rate of 10 to 30 L/min.
Patent Document 3 discloses a double-shielded-type TIG welding method wherein, a flow density of an inner shield gas ejected from an inner nozzle of a torch, which is used for the double-shielded-type TIG welding, is adjusted to a value in a certain range, and then arc is generated to an electrode so that a welding portion is welded. As an example of the inner shield gas, it is described that argon gas including 5% of hydrogen is used.
Furthermore, Patent document 3 discloses that, when the flow density of the inner shield gas is in a range of 0.1 to 0.2 L/min·mm2, variations of the penetration depth can be included in a 0.5 mm range.
Here, when the flow density of an inner shield gas is 0.1 L/min·mm2, a flow rate V of the inner shield gas obtained by a conversion of 0.1 L/min·mm2 is as follows.V=(0.1×1000×1000)/(60×1000)≈1.66(m/sec)
Furthermore, when the flow density of an inner shield gas is set to 0.2 L/min·mm2, the flow rate V of the inner shield gas is 3.3 m/sec.
In a case that TIG welding is performed for a ferrite stainless steel sheet using the aforementioned welding torch having a double-shielded structure and a penetration depth of the welding is set to become deep, it is difficult to use argon gas, which is described in Patent document 1 and includes hydrogen, as a shield gas from the viewpoint of hydrogen embrittlement and low temperature cracking. Accordingly, argon gas including helium is appropriately used for such welding.
On the other hand, a ferrite stainless steel sheet has a characteristic wherein such a sheet is low in price as compared with other stainless steel sheets.
However, when argon gas including helium described in Patent document 1 is used for an inner shield gas and an outer shield gas, there is a problem that costs for TIG welding increase since helium is expensive.
Furthermore, when argon gas including helium is used for an inner shield gas and an outer shield gas, the specific gravities of the inner shield gas and the outer shield gas decrease due to the effect of helium, and improper shielding tends to be cased. In order to inhibit improper shielding, it is necessary to supply a large amount of argon gas including helium, and therefore, there is a problem in that the cost required for a TIG welding for a ferrite stainless steel sheet further increases. In addition, Patent document 1 does not disclose a ferrite stainless steel sheet.
Patent document 2 discloses that helium gas (single helium gas) is used as a first shield gas which surrounds a tungsten electrode. However, helium has the high ionization potential.
Accordingly, it is difficult to generate arc, and therefore, the closer the mixing ratio of helium in the first shield gas is to that of a single helium gas, the more an arc temperature of a position which is directly under a tungsten electrode increases, and consumption of a tungsten electrode increases.
Accordingly, there is a problem that the cost required for TIG welding which is performed for a ferrite stainless steel sheet furthermore increases.
Furthermore, when a flow rate of the inner shield gas is set in a range of 1.66 to 3.33 m/sec as described in Patent document 3, irregular beads may be generated since the flow rate of the inner shield gas is too fast.
In addition, Patent document 3 does not disclose a flow rate of an outer shield gas at all. For example, when a flow rate of an outer shield gas is too fast, the helium concentration at the arc area does not achieve an appropriate value, but be a considerably small value.
Furthermore, when a flow rate of an outer shield gas is too slow, a bead appearance becomes poor due to insufficient shielding capacity, and oxidation and the like are caused. Patent document 3 does not disclose a flow rate of an outer shield gas at all.